Surface decarburization of carboncontaining alloys



UNITED ST Patented May 25, 1954 SURFACE DECARBURIZATION OF CARBON-CONTAINING ALLOYS wait 'i'i" OFFICE tion of New York No Drawing.Application April 16, 1952, Serial No. 282,678

4 Claims. 1

This invention relates to a method for reducin the carbon content of thesurface portions of articles composed of carbon-containing alloys.

Carbon is an important constituent in many alloys and must be present toprovide certain desired properties pertaining to strength in suchalloys. Under some conditions, however, the presence of carbon in theproportions necessary for the attainment of such properties is somewhatdeleterious to other equally desirable properties pertaining toductility. These latter properties are of greater import in the surfacelayers in certain instances, and in such cases it has been proposed thatthe surface portions of articles composed of carbon-containing alloys bedecarburized.

A specific example of a class of alloys in which carbon is an essentialconstituent is the group of high temperature alloys that has beendeveloped for use in jet engines, gas turbines and like devices wherehigh temperature operation is encountered. Broadly, such alloys areiron-base, nickel-base or cobalt-base materials containing about 10% to30% chromium, up to about 15% of molybdenum or tungsten or both, up toabout 8,% in the aggregate of columbium or tantalum, or titanium orcombinations thereof, up to 1% boron and up to 1% carbon. Forconciseness the method of this invention will be described withparticular reference to its eiiects on this broad class of materials butits usefulness is not limited to treatment of such materials.

Articles composed of these high temperature alloys have the property ofwithstanding great stress at high temperature; generally the higher thecarbon content the greater is the strength of these articles. However,in circumstances where repetitive heating, loading and cooling cyclesare encountered, service life is often disappointingly short. It hasbeen discovered that in such cases failure is caused or hastened by poorresistance to thermal shock; it has also been discovered thatsurface-decarburization of such articles improves their resistance tofailure by thermal shock. Therefore surface-decarburization of articlescomposed of these alloys is desirable.

The conventional Way of decarburizing the sur face portions of metalarticles is to heat such articles in a hydrogen atmosphere. Whilesatisfactory decarburization is thus attained, the, treatment must beconducted at quite high temperatures, and in the case of highly alloyedmate rials of relatively low melting point incipient fusion, or burningmay occur, adversely affecting the mechanical properties of thearticles. In the case of higher melting alloys, the high temperaturesrequired for treatment often cause changes in the microstructure of thealloys, also adversely affecting the mechanical properties of thearticles. The high temperature alloys in particular are subject to suchdamage.

This invention comprises a method of lowering the carbon content of atleast the surface portions of carbon-containing alloy articles bysubjecting such articles to the action of a gaseous mixture of hydrogenand hydrogen chloride at elevated temperature. With hydrogen chloridepresent, effective decarburization may be carried out at temperaturesconsiderably below those required when hydrogen alone is used. Morespecifically, in accordance with the invention an article to besurface-decarburized is treated with a gaseous mixture of hydrogen andhydrogen chloride containing 3% to 15% hydrogen chloride while beingmaintained at a temperature upwards of about 1050 C. for a period oftime sufficient to effect the desired decarburization. Ordinarily it isdesirable to use the lowest possible temperature even though at suchtemperature a longer treatment may be required. 7 For instance, in thecase of high temperature alloys of the broad class referred to above,decarburization may be accomplished by treatment for less than one hourat temperatures of 1250 to 1300 (3., whereas at 1100 0., treatment forfour hours may be required. Nevertheless the lower-temperature treatmentwill ordinarily be more desirable both from the standpoint of equipmentrequired and effect on the properties of the material treated.

In practicing the method of the invention the articles to be treated areheated in a furnace in which the atmosphere can be controlled, and dryhydrogen containing hydrogen chloride is introduced. A convenient way tointroduce hydrogen chloride into the hydrogen is to pass the gas throughconcentrated hydrochloric acid, and then dry the mixture but thehydrogen chloride may be introduced in other ways or may be formed insitu by introduction of chlorine or a chlorine compound which will reactwith the hydrogen.

Illustrative of the invention, samples of an alloy commonly used forfabrication into articles for use at elevated temperature were treated.The alloy contained 17.5% to 19.5% chromium; 14% to 15% tungsten; 9% to11% nickel; 2% maximum iron; 0.01% to 0.05% boron; 0.35% to 0.45 carbon;remainder cobalt. The samples were placed in a furnace and heated,different samples being held at diiferent temperatures in differenttests, and, while hot, were subjected to the action of a gaseous mixtureof hydrogen and hydrogen chloride. This mixture was prepared by passingcommercially pure hydrogen through concentrated hydrochloric acid andthen passing the gas through concentrated sulfuric acid to dry it. Theaverage dry hydrogen chloride content of the mixture obtained in thisway was about 6%.

By experiments conducted as just described it was found that effectivedecarburization '(i. e. the production of a substantially carbonlessskin to a depth of 0.01) was attainedat temperatures as low as 1100 C.,whereas the lowest temperature at which a comparable degree ofdecarburization was attained using hydrogen alone was in the temperaturerange l225 to 1250 C.

The effectiveness of this degree of decarburization on high temperaturealloys has been demonstrated by tests which have shown thatsurfacedecarburized articles possess much greater resistance to thermalshock than do otherwise similar but untreated articles. For instance,specimens of the alloy composition described above were subjected to thethermal shock test described in Technical Note 2037, published February1950 by the National Advisory Committee for Aeronautics. In this test awedge-shaped specimen inch thick at the thin edge is heated to 950 C.and quenched in a rack which holds the thin edge in contact with water.After quenching, oxide is removed and the specimen examined for cracks.The test is continued in a cyclic manner, and when a crack hasprogressed across the full thickness of the quenched edge, the specimenis considered to have failed. A specimen which had been decarburized inaccordance with the method of this invention by treatment at 1150 C. forfour hours underwent 173 cycles of this test before failure. Anotherspecimen treated by the method of the invention at 1100 C. for fourhours underwent 75 cycles before failure. Three untreated specimensfailed after 6, 7 and 7 cycles respectively.

An important advantage that the method of this invention has over priormethods of effecting surface-decarburization is that lower temperaturesmay be used. This is particularly important in the treatment of highlyalloyed materials which have relatively low melting points. Further, asalready indicated, the lower heat treating temperatures are advantageousin that they cause less modification of the microstructure of the alloysthan do higher temperatures, as well as from the standpoint of equipmentand fuel costs.

While the invention has been discussed with particular reference to itsutility in the treatment of high temperature alloys, it is equallyuseful for the surface decarburization of any carbon-containing alloys.

Related subject matter is disclosed and claimed in the application of F.S. Badger, Serial No. 282,677, filed April 16, 1952.

What is claimed is:

l. A method of lowering the carbon content of at least the surfaceportions of an article composed of a carbon-containing alloy whichmethod comprises heating such article for a period of time suflicient toproduce a relatively thin skin of substantially carbonless metal at atemperature upwards of 1050 C. and subjecting such heated article to theaction of a hydrogen-hydrogen chloride atmosphere containing between 3%and 15% hydrogen chloride.

2. A method of lowering the carbon content of at least the surfaceportions of an article composed of a carbon-containing alloy whichmethod comprises heating such article at a temperature of 1050 C. to1300 C. for a period of time sufficient to produce a relatively thinskin of substantially carbonless metal while subjecting such heatedarticle to the action of a hydrogen-hydrogen chloride atmospherecontaining between 3% and 15% hydrogen chloride.

3. A method of lowering the carbon content of at least the surfaceportions of an article composed of a carbon-containing alloy whichmethod comprises heating such article at a temperature of 1050 C. to1300 C. and subjecting such heated article to the action of anatmosphere composed of hydrogen and hydrogen chloride containing about3% to 15% hydrogen chloride, and continuing such treatment until asubstantially carbon-free skin at least about 0.01 inch thick isproduced on such article.

4. A method of improving the resistance of a carbon-containing metalarticle to thermal shock failure which method comprises heating sucharticle to a temperature of 1050 C. to 1300 C. for a period of timesufiicient to produce a relatively thin skin of substantially carbonlessmetal, and subjecting such heated article to the action of ahydrogen-hydrogen chloride atmosphere containing between 3% and 15%hydrogen chloride, thus lowering the carbon content of the surfaceportions of such article.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,199,418 Redmond May 7, 1940 2,437,528 Hodil Mar. 9, 1948

1. A METHOD OF LOWERING THE CARBON CONTENT OF AT LEAST THE SURFACEPORTIONS OF AN ARTICLE COMPOSED OF A CARBON-CONTAINING ALLOY WHICHMETHOD COMPRISES HEATING SUCH ARTICLE FOR A PERIOD OF TIME SUFFICIENT TOPRODUCE A RELATIVELY THIN SKIN OF SUBSTANTIALLY CARBONLESS METAL AT ATEMPERATURE UPWARDS OF 1050* C. AND SUBJECTING SUCH HEATED ARTICLE TOTHE ACTION OF A HYDROGEN-HYDROGEN CHLORIDE ATMOSPHERE CONTAINING BETWEEN3%